Esters of fatty substances are currently used in many applications as diesel fuel, heating oil, ecological solvents, base compounds for the manufacture of fatty acid sulfonates, amides, ester dimers, etc.
In the case of diesel fuel, which is today a major application of fatty substance esters, a certain number of specifications were established whose list, limits and methods belong to the EN 14214 (2003) standard currently applicable in Europe. The ester must contain at least 96.5% by mass of esters, at most 0.8% by mass of monoglycerides, at most 0.2% by mass of diglycerides and at most 0.2% by mass of triglycerides, few free fatty acids, that may be corrosive, less than 0.25% by mass of bound and free glycerin and no strong acids or metal traces at all. This requires a precise protocol to obtain the desired purity.
In the case of heating oil, it is obvious that all these specifications are not always useful and even sometimes detrimental but, the heating oil market and the gas oil market being often confused, the heating oil specifications resemble the gas oil specifications because, in France, heating oil can be used in agricultural tractors and building engines.
When manufacturing an ester from oil or grease and monoalcohol, 10 to 15% by mass of a by-product which is glycerin automatically forms, depending on the nature of the oil initially used. This glycerin is sold at a high price for various uses, but only when it is of high purity, which is obtained after intensive purification operations in specialized vacuum distillation plants.
In short, most commercial ester manufacturing methods quite readily lead to raw products (esters and glycerin) that however have to be purified in depth by means of various treatments that eventually put a strain on the transformation cost.
Thus, during the manufacture of methyl esters of fatty substances from refined oils and dry alcohol, whereas simple alkaline derivatives such as sodium alcoholates, soda or potash are commonly used as catalysts under rather mild conditions (temperature from 50° C. to 80° C. and atmospheric pressure), as can be read in many patents or publications, for example in the JAOCS 61, 343-348 (1984), a pure product that can be used as fuel and a glycerin meeting the specifications can however be obtained only after a great many stages.
If we take for example the most commonly used alkaline catalysts, we find, in the glycerin as well as in the ester, traces of alkaline compounds that have to be removed by washing and drying the ester fraction. In the glycerin phase, the soaps and alcoholates present have to be neutralized, the salts formed have to be filtered, the glycerin has to be evaporated after removing the water, unless the diluted glycerin is passed on ion-exchanging resins, prior to concentrating the salt-free glycerin. Finally, the excess alcohol always has to be evaporated and often distilled, while preventing this evaporation, especially when it is carried out in the ester phase, from causing the ester present to react with the partly dissolved glycerin, which would lead to the formation of monoglycerides.
In short, to obtain the wanted specifications for the glycerin and the ester, so many stages have to be performed that only large plants can be economically profitable under such conditions.
Furthermore, although vegetable or animal oil esters for use as diesel fuel are most often methyl esters, it is also possible to use vegetable or animal oil ethyl esters. The latter are prepared from ethanol instead of methanol, which leads, when the ethanol is of renewable origin, to a biodiesel fuel whose origin is 100% renewable, which is not the case with fatty acid methyl esters, methanol being generally obtained from fossil material.
Manufacture of fatty acid ethyl esters can also be preferred in geographical zones where ethanol is more plentiful or more readily available than methanol.